In turbomachines, especially steam turbines, the energy of a flow medium is converted into rotational energy of a rotor. For this purpose, the rotor comprises a plurality of turbine blades which are formed in such a way that the thermal energy of the flow medium is converted into rotational energy of the rotor. In the case of steam turbines, the flow medium is steam.
The turbomachines furthermore also comprise turbine blades which are attached to the casing, in addition to the turbine blades which are arranged on the rotor. The turbine blades which are arranged on the rotor are referred to as turbine rotor blades and the turbine blades which are arranged on the casing are referred to as turbine stator blades. The turbine rotor blades have blade roots via which the turbine rotor blades are fastened on the rotor. To this end, the blade roots are designed in such a way that they engage in corresponding recesses inside the rotor. The inner contour of the recess corresponds to the outer contour of the blade roots in this case. In principle, two blade root designs are known, namely the firtree design and the dovetail design.
The cross sectional contour of the blade root in the case of the firtree design has a leading region in the rotational direction of the turbine rotor blade and a trailing region in the rotational direction, which is characterized by a wave-like contour. The projections of such a wave-like contour form a plurality of anchoring teeth in the two regions. The turbine rotor blades are inserted by the blade roots into corresponding recesses inside the rotor. To this end, the rotor has a wave-like contour which corresponds to the blade root.
The rotor, together with the turbine rotor blades, rotates during operation at a comparatively high frequency of 50 Hz or 60 Hz, for example. It is also known that steam turbines are operated at higher rotational speeds for corresponding applications. At the high temperatures and rotational speeds which arise during operation, enormous thermal and mechanical loads occur. In particular, the blade roots of the turbine rotor blades are mechanically heavily loaded. It is possible that the vibrations of the turbine rotor blades, which occur during operation and are transferred onto the blade roots, lead to cracks in the turbine rotor blades or in the corresponding recesses of the rotor. If such a crack emerges, there is a high probability that as a result of crack propagation the crack increases further and in the worst case can lead to damage in the entire turbomachine if such a turbine blade becomes detached from the rotor during operation and creates damage in the casing, for example.
It would be desirable to have a blade root design in which even in the case of a crack developing the crack propagation is minimized.